1. Field of the Invention
This invention relates to integrated circuit packaging and, more particularly, to a memory subsystem encapsulated in a molded resin, where the subsystem comprises integrated circuits that are interconnected and stacked, preferably, upon a first portion (i.e., paddle) of a lead frame. The second portion (i.e., conductors) of the lead frame extend toward the first portion and receive bonding wires that are coupled to respective bonding pads on the stacked integrated circuits. As such, the second portion of the memory subsystem may form edge connectors configured as substantially planar pads extending along an outer surface of the encapsulated subsystem. The exposed surface of the edge connectors may frictionally contact an outer surface of corresponding pads arranged within a receptor of an electronic system, such that the receptor may thereby receive the memory subsystem.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
An electronic system is typically known as any device that may receive, transmit, and process electronic signals. Examples of popular electronic systems may include the personal computer, personal digital assistant (PDA), digital camera, or any other electronic-based appliance used in a consumer setting. A commonality among all electronic systems may be that they employ an interconnection of one or more circuits. Depending on the amount of integration, the circuits may be formed on a single monolithic substrate, often a silicon substrate, henceforth referred to as an integrated circuit.
Typical electronic systems may include one or more integrated circuits connected to each other by conductors. Thus, circuits within one integrated circuit may communicate with circuits within another integrated circuit. In order to protect the functionality of the circuits, each integrated circuit may be placed in a package to seal the integrated circuit from the environment. In addition to protecting an integrated circuit, a package may also help to distribute signals sent to and from the integrated circuit and, depending on the materials used, the package may also help dissipate heat that occurs during operation of the integrated circuit.
There may be numerous types of integrated circuit packages, though typically packages may be categorized as either ceramic packages or plastic packages. Ceramic packages may surround the encased integrated circuit with air, while plastic packages generally employ a resin that may fill the space between the integrated circuit and the surrounding package. Plastic packages may be less expensive than ceramic packages. Regardless of whether a package may be ceramic or plastic, there may be numerous package configurations and lead arrangements extending from the package. The package leads may serve to communicate signals between an integrated circuit and, thus, may be electrically connected to corresponding bonding pads on the integrated circuit in one of possibly three ways: wire bonding, Tape-Automated Bonding (TAB), or flip-chip attachment. Each arrangement is relatively well known and may be used in different applications based on cost constraints and the density of the integrated circuit bonding pads.
After a package may be formed around the integrated circuit, the matter of connecting one packaged integrated circuit to another packaged integrated circuit generally involves a printed circuit board or xe2x80x9ccard.xe2x80x9d A card is a rigid, substantially planar backbone element that employs one or more layers of trace conductors separated by a dielectric layer. The trace conductors may extend along one or more of layers of the card, and may connect leads of one integrated circuit to leads of another integrated circuit through vias. The printed circuit board may have plated-through holes (or vias) to accommodate downward extending leads of a packaged integrated circuit, or may simply have a square or rectangular pad on which planar surfaces of the packaged integrated circuit leads may be surface-mounted. The card may serve not only to interconnect signals between integrated circuits, but may also provide mechanical support for multiple integrated circuits arranged within a chassis of an electronic system. The card may thereby suffice to arrange the bonded integrated circuits a spaced distance from each other within the confines of the chassis.
In addition, numerous ways may exist in which to configure a card and the integrated circuits bonded to that card. For example, FIG. 1 illustrates a memory card 10 with edge connectors 12. Edge connectors 12 may be arranged on the backside surface of card 10 near a forward-leading edge 14 of card 10. According to such an example, edge 14 may be inserted through slot 16 extending through chassis 18 of an electronic system 20.
Therefore, memory card 10 may be inserted into receptor 22, which may be electrically connected to, for example, another card 24. Like card 10, card 24 may also contain printed conductors and one or more integrated circuits 26 interconnected with each other on a surface of card 24. However, in contrast to card 24, card 10 may include a specific purpose that may be universally applied to an electronic system, and may be obtainable from numerous vendors in the memory technology sector. Thus, card 10 may be a memory card, and may utilize edge connectors 12 that may frictionally engage conductive elements 28 arranged within receptor 22. In this manner, edge connectors 12 may be designed to releasably insert into receptor 22.
Card 10 is illustrated in partial breakaway in FIG. 2. Card 10 may have one or more interconnected integrated circuits 30 which may also be connected to edge conductors 12 by trace conductors 32. A memory card preferably uses a form of memory array. A popular memory array may involve an array of non-volatile storage elements. The non-volatile storage elements may be configured on a single monolithic silicon substrate to form a non-volatile memory integrated circuit 30b. Along with circuit 30b may be memory controller 30a. In addition to integrated circuits 30, card 10 may also have mounted thereon discrete devices, such as decoupling or de-bounce capacitors 34. Capacitors 34 may serve to minimize transient noise applied to trace conductors 32.
In addition to the printed circuit board (or card) on which memory 30b, memory controller 30a, and capacitors 34 may be secured, card 10 may also include covering 36. Covering 36 may surround and protect the integrated circuits and capacitors mounted on card 10. Furthermore, tab or switch 38 may be formed as part of covering 36, such that when moved, switch 38 may prevent a write operation to the memory integrated circuit. Switch 38 thereby suffices to xe2x80x9cwrite protectxe2x80x9d memory card 10. If switch 38 is activated, any signal sent to edge conductors 12 to be written onto the storage elements of memory 30b may be prevented from being stored. Activation may occur simply by moving switch 38 from one position to another along the sidewall surface of card 10.
The memory card 10 shown in the configuration of FIGS. 1 and 2 gained popularity, for example, during the advent of flash memory. Flash memory may be easily erased and reprogrammed. Once reprogrammed, the data within the flash memory is said to be non-volatile and may remain until erased or again reprogrammed. Thus, card 10 may be erased and reprogrammed while in receptor 22 provided, of course, that switch 38 is not in the write protect position. Once programmed, any data stored within non-volatile memory 30b of card 10 may remain, thereby allowing card 10 to be removed and reinserted at a later time whenever that data may be neededxe2x80x94similar to a floppy disk.
At present there are numerous types of memory cards having the aforesaid characteristics. Popular such memory cards include: Sony""s memory stick, compact flash, smart media, PC cards, flash path, multimedia cards and secure digital. All of the well-known memory cards typically include both a memory controller and non-volatile memory mounted on the card itself, or the controller may form a part of the memory interface, all of which may be interconnected to the edge connectors. As such, memory modules that include two or more die, such as a controller die and storage element, may also be called multi-chip modules (MCM).
The most common type of MCM may be the side-by-side MCM, which may mount two integrated circuits (or two die) next to each other on the top surface of a package substrate. Interconnections between integrated circuits and conductive traces on the substrate may typically be achieved by wire bonding. However, the side-by-side MCM may suffer from low package efficiency since the area of the package substrate generally increases with an increase in the number of integrated circuits mounted on the package substrate. Such an increase in package size may also increase the overall cost of the package.
Thus, a multi-chip module (MCM) may be created in which one or more die, for example, memory controller and memory array chip, may be stacked upon a package substrate to increase package efficiency. U.S. Pat. No. 6,252,305 to Lin et al. describes such a multi-chip module having a stacked chip arrangement. FIG. 3 discloses a multi-chip module 31 comprising four chips 21, 23, 25, 27 stacked upon each other and mounted to a substrate 29. Thus, the MCM comprises at least two semiconductor chips, such that each chip has a row of bonding pads formed on the active surface of the chip. However, the row of bonding pads may be disposed along only one side edge of the chip. The semiconductor chips are mounted to a substrate in a stacking arrangement, such that an upper chip is bonded to the active surface of a lower chip in such a manner that no portion of the upper chip interferes (or covers) each bond pad of the lower chip. Such an arrangement may permit wire bonding of the stacked chips to the underlying trace conductors 33 on the surface of package substrate 29.
In stacking arrangements as described above, it appears necessary to include active bonding pads arranged on only one side of an integrated circuit. Arranged on the opposing sides of the integrated circuit may be dummy bonding pads that have bond capability, yet may not be connected to internal circuitry of the integrated circuit. As such, only one side of an integrated circuit may include active bonding pads, and the other three sides of the integrated circuit may have dummy bonding pads. The dummy bonding pads may be necessary only for mechanical and assembly reasons, and may not serve to communicate with internal circuitry of the integrated circuit. Thus, the individual chips having bonding pads disposed along only one side of the chip, as described in the above prior art, may each be wire bonded to a package substrate on opposing sides of the substrate. Therefore, mounting a multi-chip module in a substrate package may allow each chip in the stack to be wire bonded to the surface of the substrate along all four sides of the substrate. However, this bonding arrangement may not be possible in other memory packaging configurations. For example, the above bonding arrangement may not be possible in memory card configurations, since memory card edge connectors (i.e. bonding pads) may be arranged along only one side of the card.
Therefore, it may be desirable to manufacture a multi-chip memory subsystem using the conventional edge connector arrangement employed by memory cards. The desired memory subsystem may, however, avoid using a printed circuit board or card for electrical routing or as a backbone for mechanical stability. The desired memory subsystem may be classified as a memory module made of less expensive materials and in less time than conventional memory cards. The desired memory subsystem may avoid the most expensive component of a memory card by eliminating the cost and lead time needed to form package material about an integrated circuit, form printed conductors upon and within a card, and form the connection between leads of the integrated circuit and printed conductors upon (or within) the card. In addition, the desired memory subsystem may integrate a memory and controller die in such a manner as to reduce the overall cost of the memory subsystem.
The problems outlined above may be in large part solved by a multi-chip memory subsystem, having the dimensions and characteristics of a conventional memory card, without the time and expense in making such a card.
In one aspect of the invention, the present memory card provides a multi-chip memory subsystem including two or more stacked integrated circuits encased by a molded resin having an outer surface on which the second end of each of the plurality of conductors terminates in a single row near an edge of the memory module. In a preferred embodiment, the stacked integrated circuits comprise a memory chip and a controller. Most preferably, the memory is a three-dimensional memory of the type described in the patents and applications listed in the cross-reference to related applications (paragraph one of this specification).
In another aspect of the invention, the present memory card provides a multi-chip memory subsystem including two or more stacked integrated circuits bonded, according to one embodiment, to a lead frame structure. The multi-chip memory subsystem preferably includes a means to couple one or more stacked integrated circuits to edge connectors in a memory card package with the capability to utilize bonding pads on all four sides of the integrated circuits.
The memory subsystem may include a stacked pair of integrated circuits, which may include storage elements and a memory controller, and any capacitive elements needed to decouple signal lines on a single monolithic substrate. The first step in processing the memory module may entail bonding conductors to bonding pads of the stacked integrated circuit similar to techniques used to bond package leads to the integrated circuit when forming a packaged integrated circuit. However, instead of merely packaging the integrated circuit, the bonded conductors may be encased within an encapsulate to form a memory module with an outer dimension similar to those of conventional memory cards. The edge connectors attributable to a memory card may be arranged in similar fashion on the memory module, where the conductors may serve not only to connect to the integrated circuit bonding pads, but also a portion of each conductor may be presented as a substantially planar surface (i.e., pad) forming a corresponding edge connector.
The memory module may include a plurality of conductors for coupling circuit elements within the memory module to circuit elements within an electronic device through corresponding conductors arranged in a receptor of the electronic device. The conductors may transmit electrical signals to and from the circuit elements via corresponding edge connectors. The conductors may also be formed having opposed first and second ends. The memory module may also include one or more integrated circuits. In this manner, the first end of the conductors may be coupled to bonding pads on one or more of the integrated circuits.
Furthermore, the second end of the conductors may be shaped similar to edge connectors of a conventional memory card. Unlike conventional integrated circuit packaging, however, the present plurality of conductors may extend in only one direction from the integrated circuit. The memory module may further include a molded resin encasing the stacked integrated circuits, and having an outer surface on which the second end of the conductors may terminate in a single row near an edge of the memory module. The row of second ends may extend flush with, or possibly extend slightly above or below, the outer surface of the molded memory module. As such, when inserted into a receptor of an electronic device, the second ends (i.e. edge connectors) may be retained only in surface contact with a corresponding planar conductive surface within the receptor. Therefore, the second ends may have a planar outer surface that releasibly connects with a corresponding planar outer surface of conductive elements within a receptor. In this manner, the memory module may be inserted and removed (i.e., released) from the receptor. During such time that the memory module may be inserted into the receptor, the row of second ends may maintain electrical communication with the conductive elements of the receptor, in order to facilitate communication between the memory module and the electronic system.
According to one embodiment, a memory module may be formed on a lead frame structure, such that the structure of the lead frame may be divided into first and second portions. The first portion of the lead frame may be adapted to receive a first integrated circuit. In addition, a second integrated circuit may be stacked upon and coupled to the first integrated circuit. For example, the first integrated circuit may be an array of storage elements (i.e. memory) while the second integrated circuit may be a controller for the memory. Alternatively, a memory die may be stacked upon a controller die, or any other combination of two or more integrated circuits may also be used. In this manner, stacking integrated circuits on a portion of a lead frame, instead of placing them side by side on a package substrate, may reduce the overall size, complexity, and/or cost of the memory module.
The second portion of the lead frame may include a plurality of conductors having opposed first and second ends. A first set of the plurality of conductors may be laterally spaced from the first portion of the lead frame. A first set of wires may extend between bonding pads of the stacked integrated circuits and the first end of the first set of conductors (i.e. conductive xe2x80x9cbonding fingersxe2x80x9d of the first set of conductors). The second end of each of the plurality of conductors may be adapted for frictional engagement with conductive elements arranged within a receptor of an electronic device. During times when the edge of the memory module may be slid into the receptor, the second end of the first set of conductors (i.e. edge connectors) may maintain an electrical connection with the conductive elements, or corresponding edge connectors, within the receptor.
In addition, a second set of wires, which transmit power and ground signals, may extend between the stacked integrated circuits and the first portion of the lead frame. The first portion of the lead frame may be laterally coupled to a second set of the plurality of conductors. In this manner, the first portion may be adapted to couple at least one of the stacked integrated circuits to power and ground connections on the second set of conductors. As such, the first portion may include power and ground planar elements. The power element may extend as a conductive ring coplanar with and laterally spaced from the ground element, such that the ground element may be the paddle portion of the lead frame. Furthermore, a first one of the second set of conductors may be adapted to connect the conductive ring to a power signal (or any other signal). Similarly, a second one of the second set of conductors may be adapted to connect the ground element to a ground signal (or any other signal). Alternatively, the ground element may be adapted to transmit a power signal and the conductive ring may be adapted to transmit a ground signal. Moreover, the first portion may be adapted to couple any two bonding pads of the plurality of conductors to bonding pads on any side of an integrated circuit.
According to another embodiment of the present invention, a lead frame may be divided into first and second portions. The first portion of the lead frame may be configured to receive a first integrated circuit. A second integrated circuit may also be stacked upon and coupled to the first integrated circuit. For instance, the first integrated circuit may be a memory while the second integrated circuit may be a controller. Alternatively, a memory die may be stacked upon and coupled to a controller die, or any other combination of two or more integrated circuits may also be used to create a stacked configuration. In addition, the first portion of the lead frame may be further separated into first and second coplanar elements.
The second portion of the lead frame may include a plurality of conductors, where a first conductor from among the first set of conductors may extend toward and connect with the first coplanar element of the first portion. Likewise, a second conductor from among the plurality of conductors may extend toward and connect with the second coplanar element of the first portion. In this manner, the first conductor may transmit a power signal to the first coplanar element, while the second conductor may transmit a ground signal to the second coplanar element. Alternatively, the first conductor may transmit a ground signal to the first coplanar element, while the second conductor may transmit a power signal to the second coplanar element. In any case, all other of the first set of conductors, except for the first one and second one, may be spaced from the first portion.
In addition, a lower surface of the first integrated circuit may be bonded to the first portion (i.e. paddle) of the lead frame in such a manner as to bond with at least a portion of each of the first and second coplanar elements (i.e. bifurcated paddle) of the lead frame. The second integrated circuit may be stacked upon and bonded to the upper surface of the first integrated circuit. Furthermore, the first portion of the lead frame may be a conductive plate, such that the first and second coplanar elements of the first portion may be adapted to transmit power and ground signals (or two dissimilar signals). The bifurcated paddle, or first portion, may also be adapted to extend beyond the dimensions of the stacked integrated circuits. Thus, the bifurcated paddle of the lead frame may be adapted to couple two dissimilar signals from the first set of conductors to bonding pads on one or more sides of the integrated circuits.
In each embodiment of the present invention, the first set of the plurality of conductors may extend into an opening created within the lead frame. The conductors, therefore, may form a part of the lead frame used in wire bonding processes. Thus, the first end of one or more of the plurality of conductors may be secured directly to a bonding pad of an integrated circuit using, for example, a wire coupled between the lead frame post, or conductor, and a bonding pad on an integrated circuit. Alternatively, integrated circuits may be placed side-by-side on a laminate-based Ball Grid Array (BGA) package, allowing routing between the two die via metal trace interconnects on the laminate material. Similar to using a lead frame package, the first end of one or more of the first set of conductors may be wire bonded to a bonding pad on an integrated circuit mounted on the BGA package. However, placing die side-by-side, rather than in a stacked configuration, may not solve the real estate reduction objective. Additionally, laminate material chip carriers may be more expensive than metal lead frame based chip carriers by an order of magnitude.
Therefore, a lead frame structure may be used, such that the lead frame post coupled to the bonding pad by a wire bond may be specially designed. The post (or conductor) may extend, for example, in two planes whereby a first plane may be above or coplanar with a first portion (or paddle) on which the integrated circuit may be secured. A second part of the conductor may be configured parallel to and below the first plane, such that the post (or conductor) may extend along the first plane downward at an angle to the second plane. According to one example, the downward angle may be less than ninety degrees, and preferably less than 60 degrees from a horizontal plane. The second part, as well as the angled joinder of the first and second parts may be encompassed entirely within a resin encapsulate. The part of the conductor that occupies the second plane may form the edge connector and, therefore, may be brought flush with the outer surface of the memory module. The part that extends along the first plane may be adapted to receive a wire bond. In this fashion, the conductor or lead frame may suffice not only to convey signals to and from an integrated circuit, but may also be shaped to extend both within the memory module encapsulate material and outside the encapsulate material.
The memory module may be formed using a simple pair of mold housings, such that the integrated circuit purposely avoids having to rest upon any mechanical support other than, for example, a first portion (or xe2x80x9cpaddlexe2x80x9d) of a lead frame. The pair of mold housings may thereby form a cavity that surrounds the lead frame-bonded integrated circuit. The pair of mold housings may include an opening, which may allow liquid resin to be inserted into the cavity. The inner walls of the pair of mold housings that form the cavity may be dimensioned according to the standards of a memory card device with associated width, height, and length configuration of a conventional memory card. Thus, the mold cavity may be much larger than the silicon substrate of an un-packaged integrated circuit. The mold cavity may form the memory module by filling the cavity with a flowable encapsulate to surround the lead frame and attached integrated circuits. In this manner, no intervening card or substrate may be necessary to support the integrated circuits, as opposed to conventional designs in which packaged integrated circuits may be mounted on a card, or flip-chip secured to a substrate that may be mounted to a card.